Sensitivity and a low-band limit of a unidirectional microphone are changed according to tension of a diaphragm. In the unidirectional microphone, the sensitivity of the microphone is increased as the tension of the diaphragm becomes higher. However, in this case, a low-band sound collection limit is shifted to a higher frequency, and thus a sound wave having a low frequency is not collected. In contrast, if the tension of the diaphragm becomes lower, the low-band sound collection limit is shifted to a lower frequency, and the sound wave having a lower frequency becomes able to be collected. However, in this case, the sensitivity of the microphone is decreased. Further, in a condenser microphone, the diaphragm is easily stuck to a fixed electrode due to electrostatic absorption force in a case where the tension of the diaphragm is low. The condenser microphone cannot collect sounds if the diaphragm is stuck to the fixed electrode.
As described above, in unidirectional condenser microphones, frequency response of the diaphragm and the sensitivity are in a trade-off relationship.
JP 2013-46194 A describes a unidirectional microphone that includes a cylindrical acoustic resistance tube in a front surface of a diaphragm to obtain favorable directional frequency response and high sensitivity.
The conventional unidirectional microphone described in JP 2013-46194 A collects a sound wave of a sound source of a low-band frequency, if the sound source exists in a position proximity to the microphone in a direction of 180° with respect to a sound collecting axis. As described above, if the sound source exists in the position proximity to the microphone, the microphone collects the low-band sound wave with emphasis. Such a phenomenon is typically referred to as proximity effect.
Directivity of the unidirectional microphone is controlled by a sound pressure difference between two points. Therefore, a typical unidirectional microphone includes two opening portions through which the sound waves are taken in, in the front and rear of the microphone. In the conventional unidirectional microphone, when the sound source exists near the rear opening portion, a sound in a low frequency deviating from the sound collecting axis is emphatically collected due to the proximity effect. Therefore, the emphasized unnecessary low sound overlaps with a sound to be collected and deteriorates sound quality.